We are investigating the physical solution chemistry of cholesterol in aqueous solvents and the thermodynamics of mixed micelle formation between cholesterol and other amphiphiles. These studies are fundamental to our major research effort involving the thermodynamic characterization of interactions between apo-serum lipoproteins, lipids, cholesterol and cholesterol esters. The binding of a variety of amphiphilic ligands to apo-HDL polypeptides (AI and AII) and to apo-LDL as well as to a number of membrane proteins is being studied together with the conformational states of the proteins in these complexes.